The invention relates to steel articles exhibiting anti-coking properties, and to a process for producing such articles by depositing an anti-coking coating on a matrix constituted by a steel, generally a refractory steel. The process is used to manufacture articles which must be resistant to coking in a variety of refining or petrochemical processes.
The coated articles of the invention can be used in a variety of refining and petrochemical processes which involve temperatures of over 350xc2x0 C.: steam reforming, dehydrogenation and visbraking, inter alia. More particularly, the invention is applicable to the manufacture of pipe stills for steam cracking intended to have a long service life at temperatures of the order of 800xc2x0 C. to 1100xc2x0 C.
The carbonaceous deposit which develops in the stills during hydrocarbon conversion is generally known as coke. This coke deposit is a problem in industrial units. The formation of coke on the walls of the pipes and reactors causes a reduction in heat exchange and major blockages and thus pressure drops increase. To keep the reaction temperature constant, it may be necessary to increase the wall temperature, which risks damaging the steel constituting the walls. There is also a reduction in the selectivity of the units and, as a result, a reduction in the yield.
It is thus necessary to stop the units periodically to carry out decoking. It is thus of economic importance to develop materials or coatings which can reduce coke formation.
Many documents have described the coke formation reaction in a variety of reactions which bring hydrocarbons into contact with walls at a high temperature. The coking phenomenon has been widely described and studied for thermal cracking of hydrocarbons in particular. The leading article can be considered to be that by professor FROMENT published in 1990 in the xe2x80x9cReview of Chemical Engineeringxe2x80x9d, volume 6, number 4, pages 292 to 328, entitled xe2x80x9cCoke formation in the thermal cracking of hydrocarbonsxe2x80x9d. A more recent title which can be cited is that by BILLAUD, BROUTIN, BUSSON, GUERET and WEILL, the first part of which was published in the xe2x80x9cRevue de I""Institut Francais du Pxc3xa9trolexe2x80x9d, 1992, volume 47, number 4, pages 537 to 549, entitled xe2x80x9cCoke Formation during Hydrocarbon Pyrolysisxe2x80x9d, and the second part of which was published in the same review in 1993, volume 48, number 2, pages 115 to 125.
To summarise the observations described in the prior art, it can be said that coke formation during thermal cracking of hydrocarbons is a complex phenomenon which involves different mechanisms, at least one of which involves reactions catalysed by the presence of oxides of metallic elements such as nickel, iron or cobalt at the walls of the apparatus used to carry out those processes. The metallic elements are generally contained in large quantities in the refractory superalloys used mainly because of the high temperatures encountered at the walls of those apparatus. That catalytic mechanism is highly preponderant: observations have shown that if that mechanism is inhibited, in the case of steam cracking it is possible to increase the length of the cycle between two still decoking stoppages necessary for implementing such a process by a factor of at least 3.
A certain number of documents are based on methods which can inhibit the catalytic formation of coke.
Japanese application P-03-104843 describes a refractory anti-coking steel for a still tube for cracking ethylene.
Further, U.S. Pat. No. 5 208 069 describes a method for passivation of the metal surface of reactor tubes coining into contact with hydrocarbons by in situ decomposition (i.e., in the assembled apparatus) of a non oxygen-containing organometallic silicon derivative under conditions in which a fine layer of ceramic material is formed on the tube surface. That method, in which deposition is carried out at atmospheric pressure or at a slight underpressure, generally does not result in the production of a deposit which is relatively uniform over the entire length of the tubes as the rate of growth of the deposit is not uniform along the entire length of the tube and thus the thickness, and the quality of the deposit, varies along the tube. Such variations entail a risk of producing very thick zones which thus have low adhesion and/or zones in which the silicon carbide deposit is of poor quality and thus of low adhesion. The pressure at whichxe2x80x94according to the examples in that patentxe2x80x94vapour phase deposition of an organometallic silicon derivative is carried out is far too high and does not permit homogeneous deposition as gas diffusion distances are much smaller than in a vacuum i.e., at a pressure of less than 10+4 Pa, for example. Further, the silicon carbide deposited is a compound with a low coefficient of expansion, while the substrate used normally has a far higher coefficient of expansion, which over time with cycles of heating and cooling, entails a non negligible risk of a loss of integrity of the silicon carbide layer at least at some points and as a result, the hydrocarbons come into contact with the superalloys which leads to an increase in the coking rate of the apparatus.
International patent application WO-A-95/18 849 describes articles, used as elements in cracking reactors, coated with a chromium-containing layer which protects against coking. The layer is applied using different plating or by painting. The chromium coatings obtained do not have sufficient resistance to coking, in particular in alternate coking and decoking cycles.
We have now discovered novel anti-coking steels, particularly for use in the applications mentioned above, which do not have the disadvantages mentioned above.
Thus the invention provides a steel article exhibiting anti-coking properties, characterized in that it comprises:
a refractory steel substrate comprising at least 0.2% by weight of carbon;
a carbon-rich diffusion barrier; and
an outer layer containing 90% to 99% by weight of chromium, characterized in that it is coated using a cementation method.
The substrate (or matrix) is generally constituted by a refractory steel, preferably containing 0.20% to 0.80% (percent) by weight of carbon.
More particularly, the steel has an austenitic grain structure at ambient temperature. A specific steel type for use in the invention is Manaurite(copyright) (registered trade mark of MANOIR INDUSTRIES).
Typical examples of steels which can be used in the invention have the main characteristics shown in the following table (the compositions are in weight %):
As indicated above, the invention relates more particularly to refractory steel articles coated by thermal chromization methods, such as chromization by cementation, in particular pack cementation or gas phase cementation.
The pack cementation process is well known to the skilled person. It has been described in a number of documents. As an example, the authors of U.S. Pat. No. 5 589 220 state that pack cementation is a process derived from CVD (Chemical Vapour Deposition) which consists of heating a pack at a high temperature in a closed or open vessel containing the metallic article to be coated for a predetermined time during which a diffusion coating is produced on the metallic article. The cementation pack in the closed or open vessel is protected from oxidation by an inert or reducing atmosphere. The cementation pack consists of a metal or alloy article or substrate for coating, surrounded by the elements to be deposited (in the form of a metal or a master alloy), a halide salt activator and a powdered filler. An inert gas such as argon or hydrogen is used as the gaseous environment for the pack. Once the pack has been heated to a fairly high temperature, the salt activator reacts with the metal or master alloy powder to form metal halide vapours. The metal halide vapours diffuse towards the surface of the metal or substrate through the gas phase of the porous pack. At the substrate surface, a reaction step occurs which results in deposition of the desired element and in the formation of a protective coating at the metal surface by solid state diffusion.
The surface reaction can be relatively complex and involves adsorption, dissociation and/or diffusion of molecular species.
Gas phase cementation consists of heating an open vessel containing a metal article to a high temperature for a predetermined period, during which a diffusion coating is produced on the metal article by a chromium halide gas generated by the action of a halide and/or its hydride on a bed of chromium or chromium alloy granules with a 0.1 mm to 50 mm diameter.
The metallising gas is transported from the granule bed to the article by a carrier gas via a specific distributor-diffuser.
The carrier gas is a gas such as argon or hydrogen, as a protective gaseous environment.
The process of supplying metallising gas to the metal article is governed by the rules of fluid dynamics and limits the formation of the chromium deposit.
A balanced choice of physical parameters (temperature, treatment period, mass flow rates of the gaseous species) and the chemical activity of the chromium halide, optimised by the ratio of the solid masses present (halide/granular chromium) in the presence or absence of its hydride, stabilises the chemical activity of the metallising halide and the coating formation conditions.
The concentration by weight of the chromium granules is at least fifty percent.
More particularly in the process of the invention, the pack cementation conditions described below are used.
The articles to be coated are placed in boxes containing a cement constituted by chromium (30% to 40% by weight) and alumina (60% to 70% by weight) powders and a halide activator (0.1% to 2% by weight with respect to the two powders together) in an atmosphere of hydrogen or argon, for example. An isothermal heat treatment is then carried out at a temperature of 900xc2x0 C. to 1200xc2x0 C.
The coated articles of the invention can in general be used as constituent materials for tube bundles for hydrocarbon pyrolysis reactors, in the presence or absence of steam, more particularly for steam reforming or steam cracking reactors. They can also be used as constituent materials for tube bundles for oil or petrochemical treatment stills, such as for visbreaking. They can also be used to coat rings and/or baffles in fixed bed reactors for petrochemicals treatments, such as dehydrogenation or reforming.
In this type of application, the coated articles of the invention exhibit improved anti-coking properties. Further, the deposits obtained do not deteriorate during severe thermal cycles between ambient temperature and 1000xc2x0 C., for example, with heating and cooling rates or 500xc2x0 C./h.
The entire disclosure of all applications, patents and publications, cited above and below, and of corresponding French application 97/08 511, filed Jul. 4, 1997 are hereby incorporated by reference.